1. Field of the Invention
The present invention relates to a device for detecting a knock of an internal combustion engine on the basis of an ionic current which flows in an ignition plug by combustion of fuel and for correcting the controlled variable of the internal combustion engine when the knock is detected. More particularly, the present invention relates to a knock control device for an internal combustion engine in which a relative reference value (background level) for knock judgement is calculated on the basis of an maximum noise level which is arithmetically estimated from a knock level signal to always optimize the background level, thereby preventing a misjudgment and a control error which are caused by the fluctuation of the noise level at unsteady time.
2. Description of the Related Art
Up to now, a knock control device for an internal combustion engine is designed in such a manner that it is judged whether a knock occurs during running, or not, and when it is detected that the knock occurs, the controlled variable of the internal combustion engine is corrected to a knock suppression side (for example, an ignition timing is corrected to a spark delay side) in response to the knock quantity in order to prevent the internal combustion engine from being damaged.
Also, in order to detect the knock of the internal combustion engine, there has been proposed a device using a change in the ion quantity which is caused by combustion of fuel.
The knock control device for the internal combustion engine using the ionic current is effective in realization of the reduced costs since the knock intensity can be detected for each of cylinders without using a knock sensor.
In the device of this type, in order to prevent the knock from being erroneously detected due to the superimposed noises of the ionic current, a noise judgement reference level (background level) is set with respect to an ionic current detection signal.
For example, in a device disclosed in Japanese Patent Unexamined Publication No. Hei 10-9108, a background level (judgment reference of the noise level) which is calculated from a sum of an average value of the detection signal intensity and an insensitive region (offset value) responsive to a running state is set with respect to a signal resulting from subjecting a knock current detection signal to a waveform shaping processing or the like.
FIG. 12 is a block diagram schematically showing a conventional knock control device for an internal combustion engine. Also, FIG. 13 is a timing chart showing the operating waveforms of the respective signals in FIG. 12, and shows a case in which a knock signal Ki is superimposed on a waveform shaping signal Fi of an ionic current detection signal Ei.
Referring to FIG. 12, an ignition unit 1 of an internal combustion engine (engine) includes an ignition coil having a primary winding and a secondary winding, and a power transistor that permits or interrupts the flow of a primary current i1 in the ignition coil (refer to FIG. 13), both of which are not shown.
The power transistor within the ignition unit 1 conducts the on/off control (permission/interruption of the flow) of the primary current i1 in the ignition coil in response to an ignition signal P from an ECU 5, and the ignition coil generates an ignition high voltage V2 from the secondary winding in response to the on/off operation of the power transistor (refer to FIG. 13).
An ignition plug 2 generates an ignition spark due to the ignition high voltage V2 which is applied to the ignition plug 2 from the ignition unit 1 and ignites an air-fuel mixture within each of cylinders of the engine at a given timing. In other words, the ignition high voltage is applied to the ignition plug 2 of each cylinder to be controlled in response to the ignition timing.
An ionic current detecting circuit 3 includes, in order to detect an ionic current that flows in a gap of the ignition plug 2 at the time of combustion, a bias means (capacitor) (not shown) for applying a bias voltage to the ignition plug 2 through the ignition coil within the ignition unit 1, and a resistor (not shown) that outputs an ionic current detection signal Ei.
A variety of sensors 4 include a known throttle opening sensor, a crank angle sensor, a temperature sensor and so on, and produce a variety of sensor signals indicative of the running state of the internal combustion engine. For example, the crank angle sensor among the various sensors 4 outputs a crank angle signal SGT in response to the r.p.m. of the engine (refer to FIG. 13).
The various sensor signals including the ionic current detection signal Ei and the crank angle signal SGT are inputted to the ECU 5 made up of a microcomputer. The crank angle signal SGT has a pulse edge representative of a crank angle reference position of each cylinder and are employed for various control operation within the ECU 5.
The ECU 5 includes a means for detecting a knock from a knock level signal (which will be described later) based on the ionic current detection signal Ei, and an ignition control means 7 for correcting the spark delay of the ignition signal P on the basis of a knock detection result from the knock detecting means.
The ignition control means 7 includes, in order to produce the ignition signal P on the basis of the running state from the various sensors 4 and a knock judgement result from a comparing means 15, an ignition timing operating means for deciding an ignition timing of the engine in response to the running state thereof and an ignition timing correcting means for arithmetically operating a spark delay quantity responsive to the knock detection quantity and allowing the spark delay quantity to be reflected on the ignition timing when the occurrence of knock is detected.
The controlled variable operating means for arithmetically operating the controlled variable of the engine is not limited to the ignition control means 7 but includes a fuel injection control means (not shown) for arithmetically operating a fuel injection amount and an injection timing, and so on. Also, the control variable correcting means for suppressing the knock can correct the spark delay of the fuel injection timing.
A knock detecting means is made up of a filtering means 11 formed of a band-pass filter, a counter means 12, an averaging means 13 within the ECU 5, an offset means 14 and a comparing means 15.
The filtering means 11, which includes a waveform shaping means, extracts a knock signal Ki having a given frequency band from a waveform shaping signal Fi (refer to FIG. 13) of the ionic current detection signal Ei. The counter means 12, which includes a waveform processing means, counts the number of pulses N of the knock signal Ki after waveform processing is performed.
The counter means 12 constitutes a knock level calculating means and calculates the number of pulses N (knock level signal) corresponding to the knock state of the engine on the basis of the knock signal Ki. The number of pulses N (knock level signal) represents the amount of knock occurrence.
The averaging means 13 averages the number of pulses N and calculates a knock level average value AVE. The offset means 14 offsets the knock level average value AVE and produces a background level BGL (noise level judgement reference).
The offset means 14 includes offset calculating means for calculating an offset value OFS with respect to the knock level average value AVE in response to the running state of the engine, and a background level calculating means for calculating the background level BGL by adding the knock level average value AVE and the offset value OFS.
The comparing means 15 constitutes a knock judging means and compares the number of pulses N (knock level signal) with the background level BGL to judge the knock state of the engine. The comparing means 15 outputs a comparison result indicative of a knock occurrence when the number of pulses N exceeds the background level BGL.
Subsequently, the operation of the conventional knock control device for the internal combustion engine will be described with reference to FIGS. 12 and 13 as well as the flowchart of FIG. 14.
First, the ECU 5 takes the crank angle signal SGT and so on from the various sensors 4 and conducts various arithmetic operations in response to the running state to output a drive signal to various actuators such as the ignition unit 1.
For example, the ECU 5 turns on/off the power transistor within the ignition unit 1 in response to the ignition signal P to permit or interrupt the flow of the primary current i1.
In this situation, a bias power supply (capacitor) within the ionic current detecting circuit 3 is charged with a primary voltage V1 generated at the ignition coil when the primary current i1 is permitted to flow.
Also, when the flow of the primary current i1 is interrupted (corresponding to the ignition timing of the engine), the primary voltage V1 goes up so that a secondary voltage V2 (several tens kV) boosted more is generated from the secondary winding of the ignition coil. The secondary voltage V2 is applied to the ignition plug 2 of an ignition control cylinder, to thereby burn a fuel-air mixture within a combustion chamber.
With the above combustion of the fuel-air mixture, since ions occur within the combustion chamber of each the combustion cylinder, the bias voltage charged in the capacitor within the ionic current detecting circuit 3 is discharged through the ignition plug 2 which has been just now ignition-controlled.
The resistor within the ionic current detecting circuit 3 converts the ionic current into a voltage and outputs the voltage as an ionic current detection signal Ei.
The ionic current that flows through the ignition plug 2 after combustion is inputted to the filtering means 11 as the ionic current detection signal Ei.
In this situation, if a knock occurs in the engine, since a knock vibration component is superimposed on the ionic current, the waveform shaping signal Fi of the ionic current detection signal Ei has a waveform on which the knock vibration component is superimposed as shown in FIG. 13.
The operation of processing the ionic current detection signal Ei is shown in FIG. 14. Referring to FIG. 14, the filtering means 11 first extracts only the knock signal Ki from the waveform shaping signal Fi of the ionic current detection signal Ei (step S1).
The counter means 12 shapes the waveform of the knock signal Ki and converts it into a knock pulse train Kp, and thereafter counts the number of pulses N of the knock pulse train Kp (step S2).
The number of pulses N greatly pertains to the intensity of knock and, as will be described later, is used for the knock judgement and also used for update operation of a succeeding background level BGL.
In other words, the comparing means 15 within the ECU 5 compares the number of pulses N with the background level BGL which has been previously calculated and judges whether the number of pulses N is larger than the background level BGL, or not (step S3).
Since the number of pulses N becomes larger as the intensity of knock becomes large, the comparing means 15 can judge the presence/absence of a knock and the intensity of the knock on the basis of the number of pulses N.
The ignition control means 7 calculates a spark delay controlled variable for making a spark delay of the ignition timing (suppression of the knock) (step S4) if N&gt;BGL (that is, YES) is judged in the step S3. On the contrary, the ignition control means 7 calculates the spark advance controlled variable for making a spark advance of the ignition timing (step S5) if N BGL (that is, NO) is judged in the step S3.
In this situation, the ignition control means 7 refers to the spark delay correction amount at the time of the previous and present ignition control in the step S4 and refers to the spark advance correction amount at the time of the previous ignition control in the step S5, to thereby calculate the respective controlled variables.
Also, if a state of N&gt;BGL (knock occurrence) is continuously judged in the step S3, the spark delay amount is sequentially integrated, and the integration of the spark delay is stopped at the time when the knock occurrence is not judged.
The background level BGL (a given number of pulses) which is a relative reference for knock judgement is set to, for example, a value of about 5 to 20 although it depends on the r.p.m. of the engine, the waveform shaping level of the detection signal Ei or the like.
In this way, if the knock occurrence is detected on the basis of the number of pulses N by the comparing means 15, the controlled variable is corrected to the knock suppression side (that is, the ignition timing to a knock occurring cylinder is optimized), thereby being capable of effectively suppressing the knock.
On the other hand, the averaging means 13 within the ECU 5 averages the number of pulses N (filter processing) and calculates the knock level average value AVE using the following expressions (1) and (2) (step S6). EQU AVE=AVE(n-1).times.KF+NP.times.(1-KF) (1) EQU NP=max{N-BGL(n-1),0} (2)
In the expression (1), AVE(n-1) is a previous value of the knock level average value AVE, and KF is an averaging coefficient (0&lt;KF&lt;1). Also, in the expression (2), BGL(n-1) is a previous value of the background level BGL.
Also, the offset means 14 adds the offset value OFS to the knock level average value AVE and calculates the background level BGL as represented by the following expression (3) (step S7). EQU BGL=AVE+OFS (3)
Finally, the ECU 5 stores the background level BGL which has been calculated through the expression (3) in the offset means 14 as the relative reference for knock judgement at the time of the succeeding ignition control (step S8), thus completing a processing routine shown in FIG. 14.
Subsequently, the knock detecting operation when the knock level average value AVE becomes an improper value in an unsteady state will be described with reference to the explanatory diagrams of FIGS. 15 to 17.
In FIGS. 15 to 17, an axis of abscissa is a time, and there is shown a case in which a running state is shifted from a knock non-occurrence region (steady region) to a knock occurrence region (for example, a transition region) and again returned to the knock non-occurrence region.
Also, in FIGS. 15 to 17, an axis of ordinate (the respective levels indicated by bar graphs) is the number of pulses N which includes the number of pulses Pn corresponding to the noise level and the number of pulses Pk corresponding to the knock occurrence level.
FIG. 15 shows a change in the knock level average value AVE with time and a steady noise component Cn with respect to the number of pulses N (N=about 1 to 2), FIG. 16 shows a change in the background level BGL with time and a steady noise component Cn with respect to the number of pulses N, and FIG. 17 shows a change in the background level BGL with time with respect to the number of pulses N in the case where a variation exists between the respective cylinders.
Referring to FIG. 15, since the steady noise component Cn is relatively stably changed at a low level and always equal to or less than the knock level average value AVE, there is no case in which the stead noise component Cn is misjudged as the knock.
However, since the number of pulses Pn of the unsteady noise component (relatively high level) exceeds the knock level average value AVE, it is misjudged as the number of pulses Pk having the knock occurrence level.
In particular, in the knock occurrence region, the number of pulses Pn of the unsteady noise component is liable to be detected, which causes the knock misjudgment, thereby being liable to mis-control the controlled variable such as the spark delay correction of the ignition timing.
On the other hand, as shown in FIG. 16, if the offset value is added to the number of pulses N to set the background level BGL high in level, it is suppressed to misjudge the number of pulses Pn of the noise level as a knock state.
However, in the case where a variation exists between the respective cylinders, as shown in FIG. 17, in a cylinder small in the detected amount of the number of pulses N, the number of pulses Pk having the knock occurrence level becomes equal to or less than the background level BGL, thereby causing the number of pulses Pk to be misjudged as the number of pulses Pn having the noise level.
As a result, even in the knock occurrence state where the ignition timing should be spark-delay corrected, the frequency of knock occurrences increases without executing the correction of the controlled variable, resulting in a fear that the engine is damaged.
As described above, in the conventional knock control device for the internal combustion engine, since the background level BGL is not shifted to a proper value depending on the occurrence state of the number of pulses N (knock level signal), there arises such a problem that the number of pulses Pn of the unsteady noise component is misjudged as the knock state as shown in FIG. 15, or the number of pulses Pk having the knock occurrence level is misjudged as the noise state as shown in FIG. 17, to thereby deteriorate the knock controllability.